CH 13
CH 13
CH 13
Operating System Concepts – 9th Edition! Silberschatz, Galvin and Gagne ©2013!
Chapter 13: I/O Systems
I/O Hardware"
Application I/O Interface"
Kernel I/O Subsystem"
Transforming I/O Requests to Hardware Operations"
STREAMS"
Performance"
Operating System Concepts – 9th Edition! 13.2! Silberschatz, Galvin and Gagne ©2013!
Objectives
Explore the structure of an operating systemʼs I/O subsystem"
Operating System Concepts – 9th Edition! 13.3! Silberschatz, Galvin and Gagne ©2013!
Un peu de français
http://gdt.oqlf.gouv.qc.ca : Grand Dictionnaire Terminologique de lʼOffice Québécois de la Langue Francaise"
http://translate.google.com/"
Operating System Concepts – 9th Edition! 13.4! Silberschatz, Galvin and Gagne ©2013!
Overview
I/O management is a major component of operating system design and operation"
Important aspect of computer operation"
I/O devices vary greatly"
Various methods to control them"
Performance management "
New types of devices frequent"
Operating System Concepts – 9th Edition! 13.5! Silberschatz, Galvin and Gagne ©2013!
I/O Hardware
Incredible variety of I/O devices"
Storage (disks, tapes, etc.)"
Transmission (network connections, Bluetooth, etc.)"
Human-interface (screen, keyboard, mouse, audio, video, etc.)"
Specialized (jet joystick/control column, etc.)"
Operating System Concepts – 9th Edition! 13.6! Silberschatz, Galvin and Gagne ©2013!
A Typical PC Bus Structure
A disk can also have"
its own local controller"
5-640MB/sec"
PCI express:"
connects to" 16GB/sec"
faster devices" HyperTransport"
25GB/sec"
connects to"
slower devices"
USB"
port"
12M-480M-5Gbps" 115K-3Mbps" 115Kbps"
127 devices" non-standard"
Operating System Concepts – 9th Edition! 13.7! Silberschatz, Galvin and Gagne ©2013!
Device I/O Port Locations on PCs (partial)
Operating System Concepts – 9th Edition! 13.8! Silberschatz, Galvin and Gagne ©2013!
I/O Hardware (cont.)
I/O instructions from processor to control devices"
Devices usually have registers where device driver places commands, addresses, and data to write, or read data
from registers after command execution"
Four registers: data-in register, data-out register, status register, control register"
Typically 1-4 bytes, or FIFO buffer"
Devices have addresses, used by "
Direct I/O instructions"
Memory-mapped I/O!
Device data and command registers mapped to processor address space"
Especially for large address spaces (graphics)"
Operating System Concepts – 9th Edition! 13.9! Silberschatz, Galvin and Gagne ©2013!
Polling
In a handshaking protocol, for each byte of I/O"
1. Host reads busy bit from status register until 0"
2. Host sets read or write bit and if write, copies data into data-out register"
3. Host sets command-ready bit"
4. Controller sets busy bit"
5. Controller reads command register, reads data-out register, executes transfer"
6. Controller clears busy bit, error bit, command-ready bit when transfer done"
Step 1 is busy-wait (or polling) cycle to wait for I/O from device"
Reasonable if device is fast"
But inefficient if device slow"
If CPU switches to other tasks, or reduces check frequency"
But if miss a cycle, data overwritten/lost"
Operating System Concepts – 9th Edition! 13.10! Silberschatz, Galvin and Gagne ©2013!
Interrupts
Polling can happen in 3 instruction cycles"
Read status, logical-and to extract status
bit, branch if not zero"
How to be more efficient if non-zero
infrequently?"
CPU Interrupt-request line triggered by I/O
device"
Checked by processor after each
instruction"
Interrupt handler receives interrupts"
Maskable to ignore or delay some
interrupts"
Interrupt vector to dispatch interrupt to proper
handler"
Context switch at start and end"
Based on priority"
Some nonmaskable (e.g., unrecoverable
memory errors)!
Interrupt chaining if more than one device
at same interrupt number"
Operating System Concepts – 9th Edition! 13.11! Silberschatz, Galvin and Gagne ©2013!
Intel Pentium Processor Event-Vector Table
non-maskable"
maskable"
Operating System Concepts – 9th Edition! 13.12! Silberschatz, Galvin and Gagne ©2013!
Interrupts (cont.)
Interrupt mechanism also used for exceptions"
Terminate process, crash system due to hardware error"
Page fault executes an interrupt when memory access error in virtual memory paging"
Suspend process, jump to page-fault handler, move process to wait queue, page-cache management,
schedule I/O, schedule another process, return from interrupt"
System call executes via trap (software interrupt) to trigger kernel to execute request"
Of lower interrupt priority compared to device interrupts"
Interrupt mechanisms are used for time-sensitive processing, frequent, must be fast, and manage different
priorities"
Operating System Concepts – 9th Edition! 13.13! Silberschatz, Galvin and Gagne ©2013!
Direct Memory Access
Used to avoid programmed I/O (one byte at a time) for large data movement that would slow down the
CPU"
Requires DMA controller"
Bypasses CPU to transfer data directly between I/O device and memory "
Operating System Concepts – 9th Edition! 13.14! Silberschatz, Galvin and Gagne ©2013!
Six Step Process to Perform DMA Transfer
Operating System Concepts – 9th Edition! 13.15! Silberschatz, Galvin and Gagne ©2013!
Application I/O Interface
I/O system calls encapsulate device behaviors in generic classes"
Device-driver layer hides differences among I/O controllers from kernel"
New devices talking already-implemented protocols need no extra work"
Each OS has its own I/O subsystem structures and device driver frameworks"
Devices vary in many dimensions"
Character-stream or block!
Sequential or random-access!
Synchronous or asynchronous (or both)"
Sharable or dedicated!
Speed of operation!
read-write, read-only, or write-only!
Operating System Concepts – 9th Edition! 13.16! Silberschatz, Galvin and Gagne ©2013!
A Kernel I/O Structure
Operating System Concepts – 9th Edition! 13.17! Silberschatz, Galvin and Gagne ©2013!
Characteristics of I/O Devices
Operating System Concepts – 9th Edition! 13.18! Silberschatz, Galvin and Gagne ©2013!
Characteristics of I/O Devices (cont.)
Subtleties of devices handled by device drivers"
For direct manipulation of I/O device specific characteristics, usually offers an escape/back door"
UNIX ioctl() call to send arbitrary bits to a device control register and data (structure) to device data
register"
Operating System Concepts – 9th Edition! 13.19! Silberschatz, Galvin and Gagne ©2013!
Block and Character Devices
Block devices include disk drives"
Commands include read, write, seek
Raw I/O, direct I/O, or file-system access"
Memory-mapped file access possible"
File mapped to virtual memory and clusters brought via demand paging"
DMA
Operating System Concepts – 9th Edition! 13.20! Silberschatz, Galvin and Gagne ©2013!
Network Devices
Varying enough from block and character to have their own interface
Approaches vary widely, and provides other communication methods, e.g., pipes, FIFOs, streams, queues,
mailboxes"
Operating System Concepts – 9th Edition! 13.21! Silberschatz, Galvin and Gagne ©2013!
Clocks and Timers
Provide"
current time"
elapsed time"
timer (to trigger an operation at a given time)"
ioctl() (on UNIX) covers odd aspects of I/O such as clocks and timers"
Operating System Concepts – 9th Edition! 13.22! Silberschatz, Galvin and Gagne ©2013!
Blocking and Nonblocking I/O
System calls can be blocking or nonblocking I/O"
Operating System Concepts – 9th Edition! 13.23! Silberschatz, Galvin and Gagne ©2013!
Two I/O Methods
Synchronous" Asynchronous"
Operating System Concepts – 9th Edition! 13.24! Silberschatz, Galvin and Gagne ©2013!
Kernel I/O Subsystem - Scheduling
I/O scheduling"
Some I/O request ordering via per-device queue"
Some OSes try to provide fairness, reorder for efficiency, handle priorities, etc."
Some implement Quality Of Service (e.g., IPQOS)"
Operating System Concepts – 9th Edition! 13.25! Silberschatz, Galvin and Gagne ©2013!
Kernel I/O Subsystem - Buffering
Buffering - store data in memory while transferring between devices"
To cope with device speed mismatch"
E.g., modem to disk, 1000x slower, needs a second buffer (double buffering) so the modem continues
transferring when disk controller copies one full buffer on its disk"
To cope with device transfer size mismatch"
E.g., large message split into small packets over the network and reassembled at the receiving end"
To maintain “copy semantics”"
E.g., the buffer to write to disk is copied from the application space to the kernel space, and it is the
kernel copy that is written to disk; more costly, but ensure consistency of the write command"
This can be accomplished also with copy-on-write command"
Operating System Concepts – 9th Edition! 13.26! Silberschatz, Galvin and Gagne ©2013!
Sun Enterprise 6000 Device-Transfer Rates
Operating System Concepts – 9th Edition! 13.27! Silberschatz, Galvin and Gagne ©2013!
Kernel I/O Subsystem - Other Tools
Caching - faster device holding a copy of the data"
Always just a copy (while buffer does not imply another copy somewhere else)!
Key to performance, as cache is consulted first instead of more delayed original data source"
Sometimes combined with buffering"
E.g., buffer for disk I/O can also be used as a cache for other reads"
E.g., disk writes are delayed/accumulated in buffer caches for efficient disk write schedules
Operating System Concepts – 9th Edition! 13.28! Silberschatz, Galvin and Gagne ©2013!
Error Handling
OS can try to recover from disk read, device unavailable, transient (network) write failures"
Simple: OS issues a retry for a re-read, re-write, re-send, for example"
Some systems are more advanced – Solaris FMA, AIX "
Track error frequencies, stop using device with increasing frequency of retry-able errors
Most error returns an error number or code when I/O request fails"
But not all OSes return the complete error codes to the application
Operating System Concepts – 9th Edition! 13.29! Silberschatz, Galvin and Gagne ©2013!
I/O Protection
Errors and protection are closely related"
Operating System Concepts – 9th Edition! 13.30! Silberschatz, Galvin and Gagne ©2013!
Kernel Data Structures
Kernel keeps state info for I/O components,
including open file tables, network
connections, character device state, etc.
Object-oriented technique"
to call the proper function"
in UNIX"
Operating System Concepts – 9th Edition! 13.31! Silberschatz, Galvin and Gagne ©2013!
I/O Requests to Hardware Operations
Tremendous number of CPU cycles for an I/O
operation"
Consider reading a file from disk for a process:"
Determine device holding file"
C: in MS-DOS, mount table in UNIX"
Translate name to device representation"
Physically read data from disk into buffer"
but first check if in cache"
Make data available to requesting process"
Return control to process"
Operating System Concepts – 9th Edition! 13.32! Silberschatz, Galvin and Gagne ©2013!
STREAMS (to read only)
STREAM – a full-duplex communication channel between a
user-level process and a device in UNIX System V and
beyond"
Operating System Concepts – 9th Edition! 13.33! Silberschatz, Galvin and Gagne ©2013!
Performance
I/O a major factor in system performance:"
Demands CPU to execute device driver code,
kernel I/O code, schedule processes"
Context switches due to interrupts stress CPU"
Data copying loads memory bus"
Network traffic especially stressful, since each
interrupt must go from one machine to the
other one through the network, and often
returns to “echo” the proper result"
OS can handle thousands of interrupts per second,
but interrupts are expensive tasks"
changes the state"
executes interrupt handler"
restores state"
Programmed I/O can be more efficient than
interrupt-driven I/O"
Operating System Concepts – 9th Edition! 13.34! Silberschatz, Galvin and Gagne ©2013!
Improving Performance
Reduce number of context switches"
Reduce data copying in memory while passing between device and application"
Reduce interrupts by using large transfers, smart controllers, polling (minimize busy waiting)"
Use smarter hardware devices to be concurrent with CPU and bus operations"
Balance CPU, memory, bus, and I/O performance for highest throughput between each other"
Operating System Concepts – 9th Edition! 13.35! Silberschatz, Galvin and Gagne ©2013!
Device-Functionality Progression
Where to implement I/O functionality?"
At application level:"
+ flexible"
+ bugs will not crash system"
+ no need to reboot/reload drivers"
- inefficient"
context switches"
does not exploit kernel data structures"
and functionalities"
At kernel level:"
+ improves tested performance"
- more challenging in complex OS code"
- must be thoroughly debugged"
Operating System Concepts – 9th Edition! 13.36! Silberschatz, Galvin and Gagne ©2013!
End of Chapter 13
Operating System Concepts – 9th Edition! Silberschatz, Galvin and Gagne ©2013!